The present invention generally relates to a method for producing flexible plates from a photopolymer sheet. Such plates are usable for producing printed matter which is printed using a flexographic, letterpress or dry-offset printing method. More particularly, the present invention relates to the step of brushing the photopolymer sheet used for making such plates, and to a device for implementing the method of the present invention.
Flexible plates are usually produced by exposing a sensitive sheet of photopolymer material to ultraviolet radiation, with the interposition of a negative film which leaves those zones which need to appear in relief on the plate when the plate is finished exposed to the radiation. The radiation hardens the exposed zones. The material of the protected surface is then removed by brushing the photopolymer sheet using flat or rotary brushes, and a solvent or an aqueous base, depending on the type of sheet.
In the method of the present invention, the exposed face of the sheet is brushed by a brush assembly made up of a row of flat brushes with essentially rectangular brushing surfaces. The brushes each undergo a movement of horizontal rotation and the brush assembly also undergoes a movement of reciprocating horizontal translation.
Methods for producing flexible plates are known from the prior art. In particular, European Patent Application No. 0,592,343, which is incorporated herein by reference and which is commonly owned, discloses an automated method for making flexible plates from a sheet introduced with the sensitive face uppermost, and an apparatus for implementing the disclosed method. The present invention constitutes an improvement to this earlier application.
U.S. Pat. No. 5,436,105, in particular, the drawings, discloses a brushing method which combines both brushing in a horizontal to-and-fro movement in a direction that is transverse to the direction of forward travel of the photopolymer sheet and rotational brushing using brushes with circular brushing surfaces. However, this type of brushing gives rise to a major drawback in that the mechanical stresses created on the photopolymer sheet by the various movements are such that the stresses oppose the uniform and precise progression of the photopolymer sheet through the brushing section.
The present invention serves to overcome this drawback by providing brushes that are offset by 180xc2x0 in horizontal rotation with respect to the immediately adjacent brush, or to the two immediately adjacent brushes in a row of brushes. Such rotation of the brush, or of the two brushes immediately adjacent to each brush, has the effect of cancelling the stresses caused by rotation of the brush on the photopolymer sheet so that the photopolymer sheet can then progress through the brushing section with the required precision.
The improvements of the present invention can both improve the brushing step described in European Patent Application No. 0,592,343 and allow the brushing step to be performed even more quickly, by performing it continuously.
Advantageously, the brushing section (which is defined by the extreme positions of the brushes throughout the brushing step) allows the photopolymer sheets to be brushed across their width, irrespective of their length. It is also advantageous that when the movement (a recipricating horizontal translation) of the set of brushes is performed in a direction that is at a right angle to the direction of travel of the photopolymer sheet, through the brushes section, the entire surface of the photopolymer sheet is brushed by at least two adjacent brushes.
Regarding the transmission of rotational movement, it is recommended that at least one crank-type transmission shaft be arranged at each end of the brushes, and that the cranks are rotated using a single motor.